The present technique relates generally to the field of lighting systems and, more particularly, to high-intensity discharge (HID) lamps. Specifically, a hermetically sealed lamp is provided with improved sealing characteristics and resistance to corrosive dosing materials, such as halides and metal halides.
High-intensity discharge lamps are often formed from a ceramic tubular body or arc tube that is sealed to one or more endcaps. The endcaps are often sealed to this ceramic tubular body using a seal glass, which has physical and mechanical properties matching those of the ceramic components. Sealing usually involves heating the assembly of the ceramic tubular body, the endcaps, and the seal glass to induce melting of the seal glass and reaction with the ceramic bodies to form a strong bond. The ceramic tubular body and the endcaps are often made of the same material, such as polycrystalline alumina (PCA). However, certain applications may require the use of different materials for the ceramic tubular body and the endcaps. In either case, various stresses may arise from the sealing process, the interface between the joined components, and the materials used for the different components. For example, the component materials may have different mechanical and physical properties, such as different coefficients of thermal expansion (CTE), which can lead to residual stresses and sealing cracks. These potential stresses and sealing cracks are particularly problematic for high-pressure lamps.
The geometry of the interface between the ceramic tubular body and the endcaps also may attribute to the foregoing stresses. For example, the endcaps are often shaped as a plug or a pocket, which interfaces both the flat and cylindrical surfaces of the ceramic tubular body. If the components have different coefficients of thermal expansion and elastic properties, then residual stresses arise because of the different strains that prevent relaxation of the materials to stress free states. In the case of a plug-type endcap, the sealed interface between the ceramic tubular body and the endcaps restricts relaxation of the components in the axial, radial, and circumferential directions. If the endcaps and seal glass have a lower coefficient of thermal expansion than that of the ceramic tubular body, then stresses may develop as the endcaps and seal glass shrink less than the ceramic tubular body during the cooling portion of a sealing process.
In addition to the ceramic tubular body and endcaps, high-intensity discharge lamps also include a variety of internal materials (e.g., luminous gases) and electrode tips to create the desired high-intensity discharge for lighting. The particular internal materials (e.g., luminous gases) disposed in the high-intensity discharge lamps can affect the sealing characteristics, the light characteristics, and the type of materials that may be workable for the lamp components and the seal glass. For example, certain internal materials, such as halides and metal halides, may be desirable for lighting characteristics, while they are corrosive to some of the ceramic and metallic components that comprise the tubular body and endcap. Again, the corrosive nature of such internal materials may be particularly problematic for high-pressure lamps, which are relatively more sensitive to potential stresses and sealing cracks.
In certain applications, such as light projection requiring good optical control, existing high-intensity discharge lamps provide undesirable light and color characteristics. For example, existing high-intensity discharge lamps often have considerable light scattering, i.e., the apparent source size is too large, and insufficient red content of the light spectrum. The light scattering or source size is expressed quantitatively as the “etendue,” while the lack of red content is expressed quantitatively by the “color efficiency” of the high-intensity discharge lamps. Both of these shortcomings limit the screen brightness of a projection system, such as a computer or video projection system.
Accordingly, a technique is needed to address one or more of the foregoing problems in lighting systems, such as high-intensity discharge lamps.